With the progress of digital technology in recent years, digitalization of an image obtained from medical X-ray imaging has been increasingly becoming a common practice. In place of X-ray imaging using a film for X-ray diagnosis that has widely been used, X-ray image capturing apparatuses using a planar detector in which conversion elements that convert an X-ray into an electric signal are two-dimensionally placed are beginning to be used. To provide data that is valuable for diagnosis from digital data (hereinafter, referred to as X-ray image data) obtained from such an X-ray image capturing apparatus, it is vitally important to connect an image processing apparatus to a subsequent stage to perform processing such as corrections of detector characteristics, logarithmic conversions, and gradient conversion on the data.
An X-ray image capturing apparatus has a characteristic that charges due to a dark current are accumulated even when image is captured without using X-ray. Thus, in addition to a charge component of an object signal derived from X-rays that have passed through an object, a charge component derived from a dark current is added to X-ray image data not yet processed by the image processing apparatus. The dark current component could cause artifacts in the image data. To reduce the influence of the dark current component, offset correction processing is performed in which an image is captured without using X-ray and the obtained offset correction image data containing the obtained dark current component is subtracted from the unprocessed X-ray image data.
However, it is known that the dark current component fluctuates depending on an operating temperature of the X-ray image capturing apparatus or time in which charges are accumulated when an image is captured and thus, it is difficult to completely remove the dark current component by the offset correction processing. Particularly under image capturing conditions in which temporal fluctuation of the dark current component increases such as image-capturing of long-time accumulation at high temperature, a region where a pixel value is zero or less may be included in measured data as a result of performing offset correction processing for a region of a low signal in image data. Since it is necessary to replace a region containing the pixel value of zero or less by a suitable positive value when a logarithmic conversion is performed in a subsequent stage by the image processing apparatus, the image signal in the region is lost, so that an artifact in a shape of the region containing the pixel value of zero or less is generated.
Under such circumstances, a technique for suppressing an artifact caused due to a loss of an image signal by removing values of zero or less by performing image processing after adding a bias value determined from the minimum value of X-ray image data on which offset correction processing has been performed to the whole image is known (for example, see Japanese Patent Application Laid-Open No. 9-168536).
However, the above conventional technique simply adds a bias value determined from the minimum value of X-ray image data on which offset correction processing has been performed to the whole image. Thus, a pixel having a value of zero or less while containing image information and a pixel (noise component) having an unexpectedly low value cannot be distinguished. Therefore, if an unexpectedly low value is added to the whole image as a bias value, a signal originally represented by a small pixel value becomes an unnecessarily large value, causing an issue of the dynamic range being significantly damaged.